Method for optimally operating co-generation of electricity and heat and optimally operating district heating power plant

ABSTRACT

A method for optimally operating co-generation of electricity and heat in which the district heating power range is divided to a lower range and a higher range is characterized in that base load electricity and regulation electricity are produced with a steam turbine operating like a condensing turbine; the lower heating power range (B) is produced mainly by heat pumps using the energy of the exhaust steam of the turbine as an energy source; peak-load power and wintertime regulation electricity are produced with a peak-load engine; and the higher heating power range (A 3,  A 4 ) is produced partially by heat pumps using said energy as the energy source and partially by the exhaust gas heat of said peak-load engine. Both the electricity and the heat are produced with a remarkably higher fuel utilization rate and significantly more electricity in relation to heat is produced than with conventional district heating power plants. The invention concerns also an optimally operating district heating power plant realizing the above method. At the initial stage of the operation of the district heating power plant, the district heating load being partial, a bigger amount of electricity may be produced of a fuel unit than at the final stage of the operation with a full district heating load. Extra peak power at short notice may be produced with the peak-load engine with a better fuel utilization rate than with the previously known solutions.

The invention is related to a method for optimally operatingco-generation of electricity and heat in which method the districtheating power range is divided to a lower range and a higher range. Theinvention is also related to an optimally operating district heatingpower plant.

The two main sectors of energy policy are power sector and heat sector.Energy sector is combining these two sectors. In the energy activitiespower production (electricity production) is combined with heating to acombined method by which power and electricity may be co-generated witha better efficiency in comparison with separate production. In thiscombination the production of electricity is given a priority to all thebenefit obtained. Electricity is made cheaper than it really is at theexpense of heat: as the fuel costs are divided on the basis of cause,the portion of electricity is about three times that of heat; as theinvestment costs are divided on the same basis, the portion ofelectricity is 15 times that of heat; and in the co-generation ofelectricity and heat only 5% of the labour costs should be allocated toheat and the rest to electricity.

Despite of this, electrical heating is still recommended in our countryby brainwashing people with false information to use electricity toheating although the specific heat consumption is the largest in the EUcountries and is in Finland 50% larger than in Sweden where the statetoday is paying a subvention to the real estate owners who abandon usingelectricity for heating.

In the co-generation of electricity and heat with a conventionaldistrict heating turbine, the efficiency varies between 85 and 50%. In asystem properly designed the annual average is about 70%.

Earlier, the efficiency of heating in separate heating of high-risebuildings was about 90%. Unfortunately, in that kind of heating,however, the more valuable portion of the fuel, the capacity to work, islost. The use of fuel to produce only heat was then the biggestdisadvantage of the energy sector.

As far as the utilization of fuel is concerned, the best known solutionuntil today is to use a power engine together with a heat pump in whichcombination fuel may be utilized in such a way that about 1.4 to 1.6 kWhheat may be obtained from one kWh fuel.

In a so called condensing power plant producing only electricity,heating capacity of the fuel is lost with the result that fuelefficiency is only about 42%.

Because the quota of district heating turbines in our country is alreadybuilt with conventional applications—in view of energy policy built toofar—(coal-fired) condensing power is the only alternative ofconventional centralized power management which during the presentdecade is able to provide additional power as firm electricity. Otheroutlines of centralized power management offer solutions which could beunder production in about a decade, at the earliest.

In the condensing power application and district heating activities onthe basis of the use of a boiler the significance of the indigenousrenewable fuel is very small due to large losses related thereto. Thelosses of the condensing power process are commonly known. The losses ofthe district heating systems are known only by a few experts. The lossesof the distribution piping are about 12 to 20% and are larger than theenergy price of the used fuel because the energy is produced with quitelow efficiency. Conventionally, a district heating system has been atemporary phase of gathering power as a solution aiming at theproduction of district heating electricity, and the district heatingactivities in the final mode thereof are given reasons by the low costof the heat as a by-product of the power production: only one half ofthe energy price of the used fuel. District heating as the activitybased on the use of a boiler is limping also in regard to boiler losses.Usually, the heat is produced with only one boiler the power of which isequal to the maximum demand of district heating. The radiation loss of aboiler is constant and about 3.5% of the rated power. The demand of thedistrict heating power varies widely in different seasons being in thewarm periods of summertime only about 8% of the maximum power, and theannual average thereof is about 30%. So, the radiation loss is about 40%in summertime and about 12% as an annual average. The annual averages ofthe losses of the system rise to about 30%, and thereby the efficiencyas a whole is often only about 70%. Moreover, the summertime loads areoften run with expensive burning oil because of the bad controllabilityof a boiler.

In the centralized power management, the only production mode ofadditional firm electricity during the decade just begun is (coal-fired)condensing power, and so the additional capacity of electricity involvesalways, due to the use of fossil fuel, carbon dioxide emissionsincreasing the greenhouse effect. Besides this, thermal load is causedto the environment by the anergy of exhaust steam the produced amount ofwhich is about double the produced unit of electricity.

U.S. Pat. No. 4,006,857 presents a method of utilizing waste heat oflarge power plants. The method in comparison with the present inventionis considered later in this specification.

An object of the invention is to provide a district heating power plantwhich operates with a principle of co-generation of electricity and heatand for which: the investment costs are lower than for a conventionaldistrict heating power plant; the fuel consumption is smaller than for aconventional district heating power plant; and the methods of controlare more extensive, faster and more easy to manage than for aconventional district heating power plant.

A method according to the invention for optimally operatingco-generation of electricity and heat in which method the districtheating power range is divided to a lower range and a higher range, andan optimally operating district heating power plant of the invention arecharacterized by features presented in the accompanying claims.

The invention and some embodiments thereof are described in greaterdetail in the following, with reference to the accompanying drawings, inwhich:

FIG. 1 presents schematically the power dividing principle of thedistrict heating power of method and optimally operating power plant ofthe invention;

FIG. 2 is a IS plot presenting a comparison between the optimallyoperating district heating power plant and a conventional districtheating power plant;

FIG. 3 is a duration curve presentation indicating a comparison betweenthe methods of the invention and U.S. Pat. No. 4,006,857;

FIGS. 4 to 9 are IS plot presentations related to various embodiments ofthe invention;

FIGS. 10 and 11 are, respectively, a IS plot presentation and a durationcurve presentation related to an example of amending an existing powerplant in accordance with the principles of the invention;

FIG. 12 is a schematic duration curve presentation of the method of theinvention; and

FIGS. 13 to 18 are schematic duration curve presentations of alternativeembodiments of rating and modular realization of the power plantaccording to the invention.

The method and the power plant of the present invention are based onutilizing the anergy of the exhaust steam coming along with the districtheating power plant as an excellent energy source for district heatingactivity with heat pump principle and high coefficient of performance.(Anergy is that part of energy which form a balance with the environmentand which, for the low temperature level, cannot be utilized directly aselectricity or heat.)

As to the steam plant part, the optimally operating district heatingpower plant differs from a conventional district heating power plantmostly in that the mass flow is expanded longer than in a districtheating turbine whereby the turbine is operating with the controlcharacteristics of a condensing turbine giving at the same time moreelectricity per fuel unit, i.e. the product for which the power plantinvestments, which are 15 times bigger than the heating investments,must be given reasons for. When enough cooling water is available forthe cooling of the steam plant part in the local area, the cooling ofthe power plant may be planned, in this respect, with conventionalapplications, whereby the power plant may, as a new type of powerplants, compete successfully with the earlier power plant solutions.However, it is then, as the conventional condensing power plants,dependent on the cooling water and so cannot be located at any place inthe countryside. In the areas where the availability of water isassured, the optimally operating district heating power plant is surelycompetitive as such a special solution, too. As a solution which may belocated freely at any place, the optimally operating district heatingpower plant will be planned with cooling of the exhaust steam by gaswhich, at least at the early stage of development, will be air. Becausethis application at this stage of the development is most promising, thefollowing list of the advantages of the optimally operating districtheating power plant is based on the consideration thereof. If a watercooling solution would be used instead of an air cooling solution, thesuperior quality in comparison with the earlier solutions would bealmost equal.

The superior quality in comparison with the earlier methods ofco-generation of electricity and heat is in the first place consistingof the following main factors:

1. Dividing the needed district heating power to subranges (FIG. 1):

a continuously operating steam power plant with higher investment costs(about 6500 FIM/kW) for providing base load electricity and controlelectricity; and

a periodically operating peak-load power plant, suitable for thispurpose, with lower investment costs (about 1000 FIM/kW) primarily forproviding winter time intermediate load power and peak-load power.

2. Almost massless open air cooling system most commonly used forcooling by condensing the exhaust steam of the steam power plant, whichcooling system offers the following advantages:

It makes possible to produce higher electrical power from a mass flowand a fuel unit than with a water-cooled turbine because the temperatureof the incoming cooling air is lower than the temperature of seawater,for example.

It makes possible to modify the heat delivery surface in comparison withthe conventional water-cooling system so that the cooling madium (air)is on the outside of and the cooled medium (condensing exhaust steam) ison the inside of the heat delivery surface, which make possible to usethe technology used in the gas heat transfer (ribbed heat deliverysurface) and to maintain a reasonable equivalent heat delivery surfacewhich is important for the air cooling.

It makes possible to use materials significantly cheaper thanconventional special-purpose brass grades for the heat delivery surfacebecause clean air and steams inhibited with the water treatment of thepower plant are not corrosive. Also, spot corrosion caused by the sludgeof a water-cooling system is not to be afraid of.

Condenser tube cleaning equipment necessary for a water-cooling systemare not needed.

Cleaning measures (a lot of work) typical for water-cooling systems arenot needed during shutdowns.

Because the cooling medium is on the outside of the heat deliverysurface, it may be installed in such a position that it is emptied bydraining and is drying of itself as the plant is withdrawn fromoperation, whereby the risk of corrosion is eliminated.

Air cooling system makes possible fast control measures of electricpower which is not possible with a conventional district heating powerplant. Especially at the initial stage of the operation, the connecteddistrict heating power being low, the control range of the electricpower is significantly larger than with a conventional district heatingpower plant. The control characteristics of the novel power plant aredetermined, like for a condensing power plant, by the thermalloadability of the turbine whereby the rates of change of the power aresimilar to those of a condensing power plant.

In view of the presentation of FIG. 2, air cooling system together withthe production of district heat with heat pumps makes possible anadditional recovery of electricity per fuel unit in comparison withproduction of electricity with a conventional district heating turbine.Arrow E1 shows the additional recovery of electricity at the final stageof the operation as the district heating power in its entirety is putinto effect. Arrow E2 shows the same at the initial stage of operationas only a part of the district heating power is put into effect, andarrow E3 shows the difference in the recovery of electricity during thecoldest period of the cold season when the temperature of the outputwater of a conventional district heating turbine is raised to themaximum.

3. The investments of a district heating power plant may be allocatedaccording to the increase of the need of district heating:

The efficiency of the presented novel power plant is about 90% in so faras cogeneration of electricity and heat is concerned, and about 36% inso far as production of only electricity is concerned. In a high-powerstation operating as a condensing turbine plant, the efficiency is about42%. As to the quality of fuel utilization the plants are competing asfollows:

90·X+(100−x)·36=100·42

wherefrom X obtains the value 11.1. This means that the presented powerplant of a new type is, in view of the utilization of fuel, morereasonable than a high-power plant as more than 11.1% of its electricityis produced on the basis of co-generation of electricity and heat. Theoperation of the power plant may be started with about 11% of the finaldistrict heat power and let the reserve be almost 90%. Thus, the budgetof the initial stage is burdened only by about 11% of the heating partinvestments and the rest are activated later. Therefore the repaymentperiod of the plant is shortened.

4. The small self-driving power of the heat pumps:

As the district heating operation is started with only partial power ofabout 11% and the additional power of wintertime district heat isproduced with a diesel driven heat pump which is, in this season,advantageous in comparison with electrically driven heat pump, the needof self-driving power for heat pumps is insignificant and so the netelectricity production of the power plant is quite high. Also theunusually high coefficient of performance thanks to the high temperatureof the energy source has a contribution to the same effect.

5. Control characteristics:

Because the controllability of the steam plant part is as good as theone of the condensing power plant and the increase of the power to thefull power is possible within about two minutes from the moment ofstarting, the total controllability of the optimally operating districtheating power plant is very good in comparison with other districtheating power plants.

6. Crisis time use:

Because the power plant proposed is able to produce the electricity ofthe region as a whole, it may be planned in such a way that it may beoperated as an independent production unit, in so called islandoperation, in the situations, e.g. crisis situations, in which there isno support from a nationwide electric network. A significant advantageduring long-time cut-offs is self-sufficiency of the fuel at least inregard to the base load electricity and heat (about 90% of the annualamount of fuel).

7. Superiority in the production of electricity.

Due to the air cooling together with the heat pump heating application,the optimally operating district heating power plant produces, incomparison with a district heating power plant operating with the sameparameter, more electricity from the same amount of fuel as follows:

at the final stage of operation as the intended final power of thedistrict heating plant is put into effect, about 21% more;

at the initial stage as about 11% of the district heating power is putinto effect, about 42% more; and

in the cold season with the limit value of the temperature of the outputwater (120° C.), about 62% more.

In comparison with a high-power district heating plant with intermediatesuperheating:

at the final stage of operation as the intended final power of thedistrict heating plant is put into effect, about the same;

at the initial stage as about 11% of the district heating power is putinto effect, about 22% more; and

in the cold season with the maximum value of the temperature of theoutput water (120° C.), about 39% more.

In comparison with the system of U.S. Pat. No. 4,006,857:

at the final stage of operation as the intended final power of thedistrict heating plant is put into effect, about 360% more;

at the initial stage as about 11% of the district heating power is putinto effect, about 400% more; and

in the cold season with the limit value of the temperature of the outputwater, about 63% more.

An explanation for the fact that the turbogenerator in the optimallyoperating district heating power plant produces more electricity at theinitial stage with not full district heating power is that the finaltemperature of the cooling air may be kept lower than at the final stagebecause the air, as the heat pump power is lower, is cooled less anddoes not reach the frosting point although the back-pressure of theturbine is lowered for providing a higher electric power.

A comparison in greater detail between the optimally operating districtheating power plant and U.S. Pat. No. 4,006,857 is made with referenceto FIG. 3. There, the rectangular area C1, C2 presents the anergy of theexhaust steam of a steam power plant which is the basis of thecomparison. The upper duration curve of the district heating describesthe way in which about 60% of the anergy of the power plant is utilizedby means of the principle following the optimal design of the districtheating power of the optimally operating district heating power plant.Because cutting of the peak is not known in U.S. Pat. No. 4,006,857, thepower peak P, which lies on the same power line as the power of theexhaust steam on the horizontal line of the rectangle, as a whole is tobe obtained from the anergy of the exhaust steam. The lower durationcurve D2 shows which portion may be utilized with, i.e. about 30% of theenergy of the exhaust steam (the area confined by the curve and thecoordinate axes). As may be found, only about one half of the anergy ofthe exhaust steam may be utilized with this principle of heat productionin comparison with the method of the optimally operating districtheating power plant (curve D1). If the same amount of electricity iswanted to be produced according to the system of U.S. Pat. No. 4,006,857as with the steam plant part of the optimally operating district heatingpower plant, thanks to the unlimited controllability thereof, the powerplant thereof is to be provided with a supplementary separate coolingsystem which removes about 70% of the anergy of the power process.Otherwise the production of electricity is lowered to far less than 30%of that which the optimally operating district heating power plant isable to produce. For this kind of cooling the water with sufficientqualities for drinking water production is too valuable, and so someother cooling medium is a precondition. The method of U.S. Pat. No.4,006,857 is quite seldom justifiable. Although said supplementarycooling could be arranged, the efficiency of the plant would besignificantly lower than the efficiency of the optimally operatingdistrict heating power plant because 1.75 times the amount of the anergyof the exhaust steam of the steam power plant derived from the fuelwould be wasted in comparison with the optimally operating districtheating power plant.

8. Differences in the investments needed:

The average investment costs per electric power unit of the optimallyoperating district heating power plant in comparison with the competingsystems:

about 58% of the investment costs of a conventional district heatingplant; and

about 45% of the investment costs of the system of U.S. Pat. No.4,006,857.

So, the anergy of exhaust steam is harmful or of no value. The use ofrenewable fuel does not increase greenhouse effect due to the ecologicalbalance. Moreover, when utilizing the anergy of exhaust steam as anenergy source of a heat pump in a small-scale power plant, the load tothe environment is kept small, especially as air cooling is used. Thisconsideration leads to the fact that a small power plant utilizingrenewable fuel always provides the locality thereof with a free ofcharge energy source, which due to the temperature level thereof is mostsuitable for running heat pumps, the utilization rate of fuel being even2 at the highest.

A small-scale power plant producing energy (electricity and heat) fromrenewable fuel does not consume (utilize) our fuel reserve, as the powerplants utilizing fossil fuel do, because new fuel is growing at the samerate. The same concerns also the combustion of municipal waste becausemore waste is produced all the time as far as this kind of consumptionis allowed. As condensing electricity is produced from fossil fuel, theefficiency is only about 35%, i.e. unnecessary load of 65% is caused tothe nature. In the optimally operating district power plant this portionis only 10 to 20% at the final stage of of the operation when thethermal load is built. Because a lot of wood is used for other purposes,the growth of the forests is always significantly larger than the amountof fuel wood, and so the amount of carbon dioxide corresponding to thenot utilized portion of 10 to 20% of the anergy emission of the powerplant is consumed in the further growth of the forests and does notincrease greenhouse effect.

Accordingly, the effect of a small-scale power plant is such that italways reduces the greenhouse effect of the fossil fuel with the portionof 65% of the electric power thereof as well as the use of fossil fuelreserve to harmful condensing power plant losses with the same amount of65%. Because no useful heat is produced in the production of condensingelectricity, the heat corresponding to the power portion of a smallpower plant obtained as useful heat must, for covering the total demandof the society, be produced separately with the boilers of a districtheating system wherein the efficiency of the process is 60 to 70%. Thissystem causes the amount of about 35% of above mentioned load. Thus, theadvantage of a small power plant in comparison with a combination of acondensing power plant and a district heating system is 65+35=100%. Thisresults in that there are grounds to consider the anergy portion of theoptimally operating district power plant as non-fuel-originated whichmeans that the whole portion thereof reduces the use of fuel otherwisewith the effect of the amount thereof, and at the same time reduces thegreenhouse effect with the amount of the corresponding power portion ofthe condencing electricity production, the portion of the usefulelectricity as well as the portion of the losses.

A novelty of this patent application is in that as the peak-road powerengine is co-operating with the heat pump which utilizes the worthlessanergy of the exhaust steam of the steam power plant as an energysource, it is always possible within the scope of the present inventionto produce an amount of peak-load electricity the energy portion ofwhich is quite small but the power is relatively high, in such a waythat only about 0.5 kWh fuel is consumed per unit of producedelectricity and heat. The applicant's Finnish patent application no.972458 “Efficient system for utilizing energy” provides generalframework for production of peak-load electricity with approximately asgood a utilization rate of fuel by utilizing the environmental anergy ina location where a suitable source of anergy is available. However, thissolution can not generally be combined with the operation of a powerplant because the only energy source which is available everywhere isthe ground, and so large a land area that it satisfies the power demandof a district power plant is seldom available. Moreover, providing theground area with systems for raising the coefficient of performance,e.g. with insulating layers, is difficult in this solution. On thecontrary, the “artificial” energy source of the optimally operatingdistrict power plant, the anergy of the exhaust steam, is always anideal solution for a power plant application, in regard to both thelocation, the quality, and the time aspects of realizing the solution.The co-operation of the peak-load power engine of the optimallyoperating district power plant with the heat pump utilizing the anergyof the exhaust steam of the steam power plant as an energy sourcediffers from the solution of FI 972458, for example, in that it is ableto utilize parts of the power plant, like a feed water tank, or acondensation water tank, or a supplementary water tank, to replaceseparately for buffer purposes built tanks of the solution of FI 972458.Thus, the buffering may be started by raising the level of the feedwater tank by means of supplementary water to the maximum thereof afterwhich the level of the condensation water tank may be raised, anddespite of that, the temperature of the isolated supplementary watertank may be raised by circulating supplementary water through aheat-exchanger. As the temperature level of the anergy of the exhauststeam is significantly higher than that of any environmental anergy, ahigher coefficient of performance of the heat pump is achieved with theco-operation of the peak-load power engine with the heat pump of theoptimally operating district power plant than with the arrangement of FI972458 with the methods of raising the coefficient of performance. Asthe optimally operating district heating power plant is run with areduced output at the initial stage of the operation, the buffer tankvolume of the peak-load power equipment may be reduced also by replacingit with the fuel drying power during the peak-load hours of electricityconsumption. Then, the tank volumes may be raised later when necessary.These facts reduce need of investments at the initial stage.

In the power plant applications, the peak-load power portion will oftenbe divided between the own peak-load power equipment of the districtheating power plant and the efficient system of utilizing energy of FI972458 exploiting environmental anergies and being provided with systemsfor raising the coefficient of performance, in such a way that theformer produces peaks with longer cycles and the latter peaks withshorter cycles.

The efficiency of the optimally operating district power plant is at theinitial stage as the district heating power is only 11% of the finalpower the same as with a big condensing power plant, i. e. about 42%,whereby it produces 21% more electricity per fuel unit than a bigdistrict heating power plant with intermediate superheating, and about68% more at the final stage as all the district heating power is putinto effect whereby it produces the same amount of electricity per fuelunit as a big district heating power plant with intermediatesuperheating.

It is known that the economical rating of a district heating turbineinvolves that the district heating power is cut at a level of about 50%of the maximum power and the power portion above this limit isrecommended to be produced with an oil-fired boiler. This kind ofinstructions are given in Tekniikan käsikirja (Handbook of Technique),for example. This kind of rating involves that the temperature of theoutput water, which at the lowest in summertime is about 70° C., isincreased with the heat exchangers of the turbine to a temperature whichis 90° C. at the highest, and the need of raising the temperature abovethis is satisfied with a boiler. In this way, the economy of a districtheating power plant is the best possible, but it produces onlyintermediate load energy and base load electricity and not at allpeak-load electricity which is often desirable. In the present optimallyoperating district power plant the production of the peak-load portionof the heating power with a boiler is replaced with a peak-load powerplant producing peak-load electricity and, with exhaust gas, peak-loadheating power which in co-operation with a heat pump converting anergyof the exhaust steam of the steam plant part to useful heat provides asystem in which the peak-load energy has significantly lower costs thanin the earlier production methods of the peak-load energy.

The experts in the district heating field also know that, in regard tocontrol characteristics, a district heating power plant is an inflexibleand often expensive solution the control range of which, moreover, is atthe lowest in the wintertime as the cost of the daily control power isthe highest. This is explained in the following.

In the design of the trunk pipework of the water district heatingsystems, flow rates of water higher than 3 m/s are not allowed. Whenselecting an economical rating for the trunk pipes the flow rates closeto this limit (e.g. 2.8 m/s) are allowed for wintertime distribution ofmaximal heating power. Thus, large changes in the flow rate are notpossible. The increase of the electric power by raising the flow rate istightly restricted. The main reason for restricting the flow rates ofwater is the stress caused by the large masses of water at the fixedpoints of the pipework. The mass of water the weight of which may bemore than several freight trains flows at a speed of about 11 kilometresper hour in the trunk pipes the diameter of which may be 0.5 to 1 meterand which sometimes make curves of 90 degrees at the street corners. Itis obvious that the forces directed to the fixed points becomeexcessively high if the flow rate is raised.

Another way of controlling electric power in district heating powerplants is to change the output water temperature to deviate from thatrequired by the need of district heating, which may cause problemsdifficult to be seen beforehand. The raise of the output watertemperature to achieve additional electric power causes that the returnwater temperature turns upwards. The raise of the return watertemperature cuses later (often with a delay of several hours) a drop ofthe electric power. The delay varies depending on several factors, likethe season (outdoor temperature level), the time of a contol actions(which time of day), the durance of the control action, and so on.Therefore, it is almost impossible to know the after-effect of a controlaction; does the drop of the power coming after a delay occur at asuitable time or at another peak consumption time of a day.

A third way used to control a conventional district heating power plant,the use of an auxiliary cooler, involves also several problems. During acold period, as the advantage available from the control would be thegreatest, the whole admission capacity of the turbine of the power plantis bound to satisfy the power requirements of the district heatingloads. The increase of the admission capacity because of the controlpower is not reasonable because it deteriorates essentially the averageannual efficiency, the main factors being the decrease of the productionof electricity per fuel unit caused by the deterioration of theisentropic efficiency of the turbine and the deterioration of theefficiency of the boiler and the decrease of the average annual power ofthe boiler which are caused by the portion of the radiation loss theabsolute value of which is defined by the rated power of the boiler andwhich may raise from the theoretical value of 3.5% sometimes even to avalue greater than 20%. The increase of the admission capacity for whichthe reasons are given by the use of an auxiliary cooler are followed byalso other factors affecting disadvantageously the efficiency of theplant, such as the increase of the consumption of the self-drivingelectricity due to the overrating of the continuously driven auxiliaryequipment. The production of the auxiliary electricity as a controlpower by means of an auxiliary cooler in a conventional district heatingpower plant competes unsuccessfully with condensing plant electricity,the main reason for which is, in addition to efficiency relateddrawbacks, the unreasonably high specific heat consumption from whichheat the major portion is lost. This fact may be expressed also bysaying that the amount of electricity obtained from a fuel unit issignificantly smaller than the corresponding one of the condensing powerprocess.

With the optimally operating district heating power plant presentedhere, auxiliary cooling is proposed in relation to special applicationsfor improvement of existing conventional power plants and utilization oflow temperature condensing power technology (e.g. That based on freonturbine technology) for producing additional electricity from the wasteheat of the auxiliary condensing steam.

In the solution presented in the above mentioned U.S. Pat. No.4,006,857, the cooling of the exhaust steam is made by water wherein thewater cooling system is technically connected with other coolingprocesses. Obviously, this kind of process is not able to compete evenwith our conventional district heating plants in regard tocontrollability. Within the scope of the method of said patent there islikely no basis for arranging competition for an independent power planttype. If the method presented is to be applied, it will probably be aspecial solution with a lot of employment which may be only asupplementary unit of a company carrying on energy business. The methodpresented here, the optimally operating district heating power plant, isin all the embodiments thereof able to compete as an independent unitwith other modes of the energy production. The solution of U.S. Pat. No.4,006,857 differs essentially from the operation principles of theoptimally operating district heating power plant. In said method, theraw water with a quality of domestic water conducted through thecondenser of a power plant is transferred by means of a distributiongrid via heat consumption points to the drinking water supply plant anda portion thereof to process cooling points. The heat pumps are locatedin the vicinity of the heating points and not at the power plant like inthe optimally operating district heating power plant. In the lastmentioned plant, the whole district heating portion produced with heatpumps is extracted from the anergy of the exhaust steam of the steampower plant and is transferred in the form of heat produced by heatpumps to a closed district heating grid which is realized with knowntechnology and is operating in a conventional way, whereby thedistribution of heat is carried out with an inhibited water circulationprovided with deoxidation. In relation to this, a novelty in the presentinvention is the source of energy (the anergy of the exhaust steam). Allthe other items concerning the operation of the heat distribution gridinclude known prior technology which is proved to function. U.S. Pat.No. 4,006,857 does not consider material questions, or the question ofthe point in the process where the water is handled, or the question howmetallic parts, fixtures, balancers, e.t.c are prevented from corroding.If the raw water is treated and inhibited to be suitable for thedistribution network, it is no more suitable for drinking water. If itis not treated, there must be no parts in the distribution networksusceptible to corrosion. In view of the cooling considerations, it isnot reasonable to conduct the cooling water via the condenser. Moreover,the raw water suitable for drinking water is too valuable to be used forcooling because the availability of this kind of water is normallyrestricted, especially if groundwater is concerned. For providing onemegawatt of electric power in a condensing power plant, about 72 cubicmeters cooling water per hour is needed which means that a small powerplant of 10 MW uses about 633,000 cubic meters cooling water annually.This corresponds to the annual domestic water consumption of about21,000 households. The electric power demand of such a community isabout 170 MW. In the climatic conditions of Finland the demand ofheating power of such a community were about 300 MW. If this amount ofheat were produced by district heating, the electric power of thedistrict heating power plant were about 100 MW. These calculations showthe category in which the system of U.S. Pat. No. 4,006,857 iscompeting. A lot of development should be done to make the ideafunction, and it is always a special solution without any commonapplicability. Claim 1 of the mentioned patent document claimsdefinitely that the cooling of the condenser is carried out by waterwhich means that air cooling is out of question. Also, the location ofthe heat pumps is defined to be in the vicinity of consuming pointswhich excludes the possibility of optimizing the production of heat inaccordance with the principle of cutting the power range and with theuse of a peakload power plant in co-operation with heat pumps utilizingthe anergy of the exhaust steam of the steam power plant as a source ofenergy. For achieving a competitive position in regard to the optimallyoperating district heating power plant, the peak-load power should beproduced by applying FI 972458 (Efficient system for utilizing energy)in which, instead of utilizing the exhaust steam of the steam powerplant of the optimally operating district heating power plant, anergy ofthe environment from any available source of energy is utilized asprocessed to heat together with the exhaust gas heat of the peak-loadpower plant.

It is well-known that the process efficiency of the condensing powerplant producing basic electricity is only 35 to 42% because “thecondensing heat is wasted” (the anergy passing away from the condensercan not be utilized). It is also well-known that the efficiency of a socalled back-pressure turbine plant is better, i.e. about the same as theefficiency of a boiler producing only heat. However, producingelectricity with the back-pressure method gives the remarkable advantagethat the process efficiency is thought to be better than with condensingelectricity production also in regard to the more valuable product ofthe process, i.e. the electricity. Thus, applying these principles,electricity may be produced as follows:

In an industrial back-pressure power plant with an average processefficiency of about 85% beacause it is driven almost all the year withfull power.

In a district heating power plant with an average process efficiency of65 to 75% depending on the rating of the plant. In a properly rateddistrict heating turbine plant the process efficiency is at the bestabout 85% as the turbine is driven with full power about 1,500 hours peryear during the coldest wintertime.

Moreover, it is well-known that the other product of the districtheating power plant, i.e. heat, is transmitted to a district heatinggrid with heat-exchanger technique whereby the temperature of thedistrict heating water is raised by condensing the steam taken from theturbine for this purpose. The temperature of the circulated water israised usually from a temperature of 45 to 50° C. This means that thetemperature of the water returning to the power plant is about 50° C.When optimizing the heat delivery surfaces and the space questions ofthe heat-exchangers of the subcentres of the district as well as theradiator network of the secondary circuit of the heating systems, thelowering of the temperature of the return water essentially under thislevel is not considered reasonable. Therefore, in the art of power plantengineering the portion of the energy the temperature of which is under50° C. is considered to be of no value, so called anergy. (The conceptanergy means that part of the energy which because of the lowtemperature level thereof can not be processed to electricity or heat orwhich because of the small amount is not worth of investments forutilization.) Let us keep in mind that a higher temperature of thereturn water is only disadvantageous for the power plant process becausethe production of electricity is reduced for the impaired vacuumachieved in the condenser because of the raise of the temperature of thereturn water.

It is also well-known that the above temperature level of 45 to 50° C.is considered to be too low for heating residential or office buildingswith normal central heating systems. The rated temperatures thereof arenormally 90/60° C. or 80/60° C. As anergy of the above temperature levelof about 50° C. or a lower level is concerned for heating, thetemperature level is usually raised with heat pumps to the area of thementioned normal operating temperatures. This anergy, although it may bea good source of energy for heat pump applications, is thus to beconsidered to be of no value, and it attains the new value thereof fromthe driving power of a heat pump.

On the other hand, it is also known, especially in other countries, thatthe energy products of a power plant have different prices. So, theelectricity, the production of which needs the most processing, is themost expensive, the next expensieve is industrial steam, district heatis the cheapest, and the part which is at so low a temperature levelthat it can not be utilized as electricity or heat is considered to beof no value (=anergy). Although the anergy passing away from thecondenser is to be considered to be of no value in view of the powerplant process, which is shown also by the fact that there is no pricefor it, from the point of view of the heating sector it is, however,quite valuable although always free of charge. This is valid becausethis kind of relatively warm anergy is a good energy source for heatpumps while, on the other hand, it is detrimental from the point of viewof both the power plant and the surroundings.

However, it is also known that anergy may be processed to heat and bytoday's technology partially to electricity, too. Processing to heat maybe carried out by heat pumps, for example, and to electricity by meansof a low temperature engine like the one based on Rankine cycle oforganic fluid and so called Boost Energy Converter tecnology. From therecent steam boiler applications it is also known that the combustionair of a boiler may be heated by a heat pump whereby the anergy of thecombustion gases is processed for this purpose by cooling the gas flow.So, the efficiency of the boiler is improved.

Furthermore, it is known that in the development of an industrialcondensing power plant the most of the investment costs are related tothe industrial process, and the need for further investments forelectricity production is quite small, which results in that industrialcondensing power is the cheapest way of producing electricity. Moreover,it is known that electricity production with an industrial condensingpower plant does not cause pollution as the pollutants are related tothe industrial process itself.

It is also known that in the countries with warmer climate a remarkablyworse vacuum is obtained in the condenser of a condensing turbine thanin Finland wherein the annual average temperature of seawater is low,about 6 to 6.5° C. In Central Europe, for example, seawater is suitablefor swimming also in the coldest periods of the year, the lowest outdoortemperature being about +20° C. Of the same amount of fuel morecondensing power is produced with seawater cooling in Finland than inCentral Europe.

Moreover, it is known that in the community planning the sites for powerplants are often selected according to the availability of coolingwater, among other things. As to the siting issues, also transportationof fuel is taken into account in the centralized power maintenance. Forthese reasons the sites of the large power plants are often port cities.Accordingly, also the industries settle down at the same places. Thesefacts cause that the employment is also centralised in these areas withthe consequence that the development of the countryside is suffering andthe countryside is depopulated. This is very disadvantageous for asparsely populated country the significant natural resources of which,however, are located in the countryside.

The cooling of the optimally operating district heating power plant isalternative and free from previous issues related to the availability ofseawater. Thus, these small power plants may be located in any districtin accordance with decentralized energy production mode which givespossibilities especially for employment and development of economic lifein the counryside. At the same time the trade balance of the country isimproved and the indebtedness is reduced as the development of one usedenergy unit with indigeneous fuel reduces energy import by about fiveunits.

As the outside air is used for cooling the condenser of a turbine, thetemperature of the air is varying within the range of about +25 to −30°C. The most common rating for the outside air temperature will be about−10 to 0° C., depending on the heating system solutions for the districtas a whole. Then, the exhaust anergy of the turbine is processed by heatpumps to suit for the district heating system. The temperature of thesteam discharging from a turbine to a condenser is about 36° C., and theamount of air is selected so that the temperature of the air coming outof a condenser before a heat pump is about +10° C. as the temperature ofthe outside air is −10° C., for example. Then, the air coming from acondenser may be cooled about 10° C. The outside air being warmer thanthis for about 7,000 hours per year, the temperature differenceconcerned is significantly bigger. Thus, the heat pump will be operatedwith a high coefficient of performance. Due to this, the heatingsolution is very competitive, and this is improved further by theco-operation with a power engine, a diesel engine, for example. Withinthe outside temperature range of +10 to −30° C. heating system andenergy production system is supplemented with co-operation of productionof peak-load electricity with a diesel engine and a heat pump and byapplying recovery of the heat of exhaust gases of the diesel engine.

In the district wherein the need for heating is quite small in relationto the power of a small power plant, previously known cooling towertechnique may be applied for cooling of the condenser of a turbine. Theinfluence of this solution to the greenhouse effect is minimal incomparison with the influence of excessive carbon dioxide release causedby wrong energy policy as the use of electricity for heating isone-sidedly favoured and supported which increase the carbon dioxiderelease to six times the amount which is necessary, with the bettermethod provided by the present invention, for example. In the climaticconditions of Finland even the cooling tower solution gives moreelectricity per fuel unit than the seawater cooling in Central Europe.

Because the turbine is driven with a greater annual production than aconventional district heating turbine, the annual process efficiency isimproved from a value of 65 to 75% of a conventional district heatingturbine to a value of 85% which is approximately the same as with anindustrial back-pressure turbine. Because anergy is produced instead ofheat and this anergy is processed to heat by a heat pump with a highcoefficient of performance, the annual average of the fuel efficiency iseven further increased.

Special Applications of the Optimally Operating District Heating PowerPlant

In view of today's knowledge, the basic solution of the gas coolingsystem of the steam plant part of this power plant is that coolingmedium is outside air and the cooling is carried out with heat exchangertechnique. In the future, there may be alternative solutions becauseother gases exist which are able to compete with air technically andperhaps also economically. Besides this kind of cooling, alsoconventional cooling tower solutions may be considered, however withcertain limitations, and certain alternative modifications in whichlimitations caused by vapourizing water for heat pump use are eliminatedor reduced either by changing freezing characteristics, for example. Acommon advantage of all the gas cooling applications, however, are goodcontrol characteristics due to open massless system. Controlcharacteristics provided by the peak-load power engine part of theoptimally operating district heating power plant are definitely betterthan those of previous district heating power plants, as well as thereduction of investment costs therefore, that besides the renovations ofthe existing plants, also in the realization of new plants on the basisof these ideas cooling of the steam plant part with other ways, evenwith water, may be considered especially when some specific conditionsare in favour of that.

Special Applications of the Optimally Operating District Heating PowerPlant

1. Optimally operating district heating power plant, the steam plantpart of which operates as a so called industrial intermediate loadenergy condensing power plant which provides good prerequisites fordistrict heating activity by using heat pumps which utilize anergy fromthe cooling air flow of the condenser.

The obligation of treating the waste set by EU directives for themunicipalities gives an opportunity to utilize combustible waste by acondensing power plant incinerating waste. This kind of plant is bestjustified if it is used only in wintertime for producing so calledintermediate load energy during the period in which the price ofelectricity is the highest. Then, a plant placed in the countryside atthe same time compensates unemployment because the unemployment ishighest in the countryside in wintertime. The anergy of condensing powerplant may then be used as an energy source of a heat pump in districtheating operations. Domestic renewable fuel, like wood chips or rapeseedoil, may be used as an auxiliary or additional fuel.

2. A conventional district heating turbine is changed in such a way thatit is provided with a peak-load power plant part characteristic to theoptimallly operating district heating power plant, and a steam powerplant part of which produces with added use of auxiliary cooler baseload electricity and intermediate load energy and daily control powerfor requirements of industry, instead of producing electricity for spaceheating as is presently done. Then, the value of the auxiliary coolingheat, however is that high that it makes a small cost, about 0.015 FIMper kWh, in a plant wherein the fuel is coal or peat. If the wholecapacity of the district heating turbine is in this way generated toelectricity by means of auxiliary cooling, the amount of auxiliarycooling in a properly designed district heating plant is about 40% ofthe base load portion of the heating energy. Then, the supplementaryincome from the electricity is significantly larger than the extra costof fuel caused by the auxiliary cooling. During the coldest wintertime,a further improvement may be to produce heat for the upper part of thebasic heating load by a heat pump the energy source of which is thereturn water of the district heating grid which in this time of yearoften is too hot (about 60° C.). Another improvement may be torecirculate the least productive mass flows of the energy process(preheating steam of the feed water) through a small turbine to the sameheating purpose as other heat produced by heat pumps, for example. Thelatter improvement is natural in connection of the renovation of theboiler plant.

3. An oversized district heating turbine plant is changes so that it isprovided with a peak-load power plant part characteristic to theoptimallly operating district heating power plant and, as means forimproving the economy, with an application of low pressure condensingpower plant technology, like the Boost Energy Converter equipment basedon Rankine cycle of organic fluid. Then, either the district heatingmass flows may be increased by converting production of district heatfrom the steam to production of electricity by means of a Boost EnergyConverter or the district heating plant may be changed so that it willproduce only base load electricity so that the low pressure chamber isreplaced by a one with longer expansion suitable for condensing powerproduction, after which the district heat is developed by heat pumpspartially of the anergy of the power plant and partially of the exhaustgases of the peak-loed engine. Then, also above mentioned recirculationof the least productive mass flows throug a small turbine may beapplied, in this case to the condenser of the turbine.

4. A condensing power plant is changed so that it operates in accordancewith the characteristic features of the optimally operating districtheating power plant and is provided with a peak-load engine of suitablesize considered on the basis of the district heating demand of the localarea together with utilizing the anergy of the exhaust steam of thecondenser as an energy source for heat pumps. If the temperature levelof the anergy leaving the condenser at a temperature of about 17 to 18°C. is raised with a heat pump back to a temperature level suitable forheating, it may be utilized. Then, also the disadvantage caused to theenvironment by the cooling water released to water system may be rducedor totally eliminated. The temperature level of the anergy of a powerplant is always remarkably higher than any other natural anergy sourceavailable in Finland, like soil or water system. In geothermal heat pumpapplications the temperature of the soil is often lowered to −3° C., andwith water systems as an anergy source, the temperature level drops toabout +1° C. as the average coefficient of performance is equal to 2.4.In the power plant applications, as the temperature level of the anergyis the lowest with seawater cooling, the annual average temperature ofthe seawater being about +6.5° C. and the temperature of the exhauststeam being 18 to 20° C., the temperature of the cooling water leavingthe condenser is about 15 to 17° C. Even this outflow of the anergy ofthe cooling water provides a chance to use a heat pump with a goodcoefficient of performance if the heat pump is designed to cover thebase part of the heating system whereby the temperature of thecirculating water is raised to about 55 to 60° C. For raising furtherthe temperature of the circulating water the heat of the exhaust gasesof the peak-load engine may be used, and further on the least productivemass flows of the energy process, e.g. preheating steam of the feedwater, discarded through the renovation measures of the power plant, maybe used to cover the intermediate area of the district heating demandbetween the base part and the peak part. In the boiler disign of todaythe final heat content of the exhaust gases is utilized more efficientlythan earlier the consequence of which is that the preheating system forfeed water, so called high pressure bled steam system, used in olderpower plants is no more economically justified.

FIG. 1 presents the duration curve D and the power cutting principle ofthe optimal rating of the district heating load wherein the peak part Hof 50 percent is conventionally produced with an oil-fired boiler andthe lower part of power (base part) L is produced with a districtheating turbine by producing at the same time also electricity in theform of so called intermediate load energy and base-load electricity. Inthe optimally operating district heating power plant the base load andthe peak load are produced in the way presented with reference to FIG.12.

FIG. 2 presents how much more electricity the optimally operatingdistrict heating power plant produces per mass flow unit (and also perfuel unit) than a district heating power plant operating with the sameparameter of steam. The arrow E1 indicates the difference at the finalstage of operation as the whole district heating demand is put intoeffect, the arrow E2 indicates the same at the initial stage ofoperation as about 11% of the district heating demand is put intoeffect, and the arrow E3 indicates the difference during an extremelycold period as the district heating power plant operates with maximumtemperature (120° C.) of the output water. The arrow E4 in the upperpart of the figure indicates which additional electric power may beachieved per mass flow unit by means of intermediate superheating as thesize of the power plant is increased.

FIG. 3 presents a comparison between the methods of the optimallyoperating district heating power plant and U.S. Pat. No. 4,006,857.Because the latter does not present cutting of district heating powerfor optimizing the operation, the whole district heating power (maximumpower P) must be obtained from the power of the exhaust steam of thepower plant (the horizontal line C1 going through the point P). Thevariation of the heating demand which may be satisfied with the methodis presented by so called duration curve used in the power sector todescribe this variation. The area confined by the curve D2 and theco-ordinate axes describes which portion (about 30%) of the annualenergy corresponding to the energy of the exhaust steam (the areaconfined by horizontal and vertical line C1, C2 and the co-ordinateaxes) the plant is able to satisfy. Because in the optimally operatingdistrict heating power plant the district heating power is cut at thepoint of about 50% for optimizing the economy of the operation and theupper part of the district heating power is produced mainly with theexhaust gas heat of the peak-load engine, the district heating operationwhich may be carried out with this plant complies with the durationcurve D1. As may be seen, this curve confines about 60% of the area ofthe rectangular which means that by this method the amount of the anergyof the exhaust steam of the power plant which may be utilized is aboutdouble the amount of the competing method. Because about 70% of theanergy of the exhaust steam remains useless, the power plant must beprovided with a separate auxiliary cooling system for conducting theanergy away and for producing the same amount of electricity of the sameamount of fuel than with the optimally operating district heating powerplant. Otherwise, the electricity generation is remarkably less that 30%of the electricity generation of the optimally operating districtheating power plant.

FIG. 4 presents, by means of an expansion curve drawn on an IS plot, asituation in which an oversized district heating turbine must be drivenwith a partial load in wintertime. The upper less steep part of thecurve describes the operation of a partially admitted regulation wheel.Normally, the steam is expanded to the point P4 but for oversizing theexpansion extends to point P2. In a normal case, the district heat isproduced at two stages (with two heat-exchangers) whereby the heating atthe first stage is carried out by steam taken from the end of theexpansion (P2), and the heating at the second stage is carried out by atapping of the turbine (P1). These steam mass flows are normallyapproximately equal. Calculation of the electricity given by the turbinemay then be simpified by assuming that the whole district heating massflow is taken from point P3 the enthalpy of which is the average ofpoints P1 and P2. By such a calculation, it may be seen that that theturbine concerned gives (815−630)/860·0.98=0.211 units electricity ofone mass flow unit.

FIG. 5 indicates how a properly designed district heating turbine isdriven with full power in wintertime, but the driving scheme is wrong.The exhaust steam comes out from the turbine at a temperature of 120°C., althoug it would be better to take it out at a temperature of 90° C.and raise the temperature of outgoing water from 90° C. to 120° C. bymeans of a boiler, for example. By the above calculation, the turbineconcerned produces (815−611)/860·0,98=0.232 units electricity.

FIG. 6 indicates how a properly designed district heating turbine isdriven with partial power in summertime whereby the temperature of thedistrict heating water is raised to 70° C. The heating is now at onestage by steam taken from the end of the expansion (P2). The turbineconcerned produces (815−587)/860·0,98=2.60 units electricity.

FIG. 7 indicates how a properly designed district heating turbine isdriven with a wintertime load in a right way whereby the temperature ofthe exhaust steam of the turbine is 90° C. It may be seen that theturbine gives (815−582)/860·0,98=2.66 units electricity.

FIG. 8 presents an expansion curve of a conventional condensing powerplant with seawater cooling in the conditions of Finland. It may be seenthat the expansion curve is longer than the curves of FIGS. 4 to 7. Theconventional condensing power plant gives (815−508)/860·0,98=3.50 unitselectricity but not at all heat.

FIG. 9 presents the way in which the optimally operating districtheating power plant according to the invention is designed. It may beseen that with a small power plant in accordance with the invention,with the steam conditions: 60 bar, 510° C, and the isentropic efficiencyof 90%, (815−532)/860·0,98=3.22 units electricity is obtained, i.e.almost the same amount as with a condensing power plant. At the earlystage of operation, the district heating power being only 11% of thefinal power, about (815−490)/860·0,98=3.70 units electricity, i.e. morethan with a condensing power plant (see above: 0.350 units), may beproduced per one fuel unit with an air cooling solution due to lowtemperature of the air and a better vacuum than with a condensingturbine with seawater cooling.

FIG. 10 presents the expansion curves taken from the acceptanceinspection protocol of the back-pressure heat district turbine of stageIII of the Kokkola power plant owned by Imatran Voima Oy. The arrow E5indicates the present annual average of electricity generation, thearrow E6 the proposed proposed generation of 52 MW, and the arrow E7 theefficiency amendment achieved by the changes proposed in thisspecification for an oversized district heating turbine plant. Theacceptance inspection was carried out with artificial heating loadsbecause of the oversizing of the turbine plant. The lowest guaranteepoint was selected in view of the summertime power levels for which itcould be driven under real operating conditions (the first curve fromthe right). The summertime expansion curve came to prove the quality,also the wintertime quality, of the turbine for the whole futurethereof. Today, at the turn of the year 1998/1999, this practically idlerunning turbine, the designed power of which is 52 MW and the maximumpower with which it is run during 19 years being 40 MW, is running withthe electric power of 10 MW, the same as the starting level in 1978, asthe city of Kokkola has, for too high a price, denounced the treaty ofbuying heat from the plant.

In FIG. 11, the curve D3 presents the operation level achieved by theabove stage III turbine in 1997, 18 years after the start-up. The curveD4 indicates a proper design. The area A1 corresponds to the auxiliarycooling power, and the area A2 corresponds to a supplementaryelectricity generation achievable by a Boost Energy Converter orequivalent.

FIG. 12 presents the principle of heat production of the optimallyoperating district heating power plant. The upper hatched area A3indicates the heat originated from the exhaust gases of the peak-loadengine. The lower hatched area A4 indicates the heat obtainable from theexhaust gases of a driving engine of a heat pump of a period of a higherelectricity tariff of wintertime, and the areas without hatchingindicate heat originating from the anergy of the of the steam powerplant part. The arrow WT indicates the time of winter tariffs and thearrow ST the time of summer tariffs. The line C3 indicates the normaloptimal design power of the exhaust steam of the steam power plant partand the line C4 indicates lowering effect of the sensitivity analysis ofthe investments to the optimal design power. In the optimally operatingdistrict heating power plant, the base load B of the heating demand isproduced by heat pump technology of the anergy of a turbine with alonger expansion operating like a condensing turbine instead of aconventional district heating turbine, the turbine producing mainly baseload electricity and regulating power. The peak-load power A3, A4, A5 isproduced partially with heat pumps of the same anergy of the exhauststeam and partially of the exhaust gas heat of a peak-load engineproducing mainly peak-load electricity and wintertime regulating power.Moreover, a part the wintertime heating demand is produced of theexhaust gas heat of a possible disel driven heat pump. The portion ofthe exhaust gas heat varies from year to year mainly for outdoortemperatures and lasting of temperature periods which affect the peakload demand and so the driving times of the peakload engines. In a coldwinter, the portion of the exhaust gas heat is larger than in a mildwinter. Because the cold days are not always in succession but there maybe mild days therebetween, by storage containers of a buffer system maybe achieved a situation that by discharging heat from the storagecontainers in cold days the utilization power of the anergy may be abovethe power of the anergy C4 of the steam power plant corresponding to theportion A5 of the anergy. As the power plant concerned starts theoperation with a small part (about 11%) of the final district heatingdemand, the investments to the district heating side tolerate well thatthe operation in regard to the heat pump power is secured with an extraelectrically driven backup pump unit whereby a possibility tocompetitive bidding of driving power for heat pumps is provided.Accordingly, the area of the lower hatched area A4 may vary depending onthe competition conditions. If the upper area of the base load of thedistrict heating demand, without the exhaust gas heat, is produced withan elctrically driven backup heat pump, the plant utilizes a littlegreater part of the free anergy of the exhaust steam.

As the optimally operating district heating power plant is a small-scaleplant, by a good integrated community planning also such applicationsmay be found in which the utilization of fuel, in comparison with thehigh-power stations, causes significantly smaller release of carbondioxide or no release at all. This is achieved if the power plant isplaced adjacent to an industrial area where also industries with highdemand of cooling, like a slaughterhouse or an ice-cream factory orequivalent, may be placed. Then, the cooling with conventional electricrefrigeration machines may be raplaced by carbon dioxide cooling as theexhaust gases of the boiler are cooled to so low a temperature thatcarbon dioxide is liquefied. Thus, by means of the anergy removed by thecooling also the temperature level of the combustion air of the boilermay be raised by heat pumps. Liquid pressurized carbon dioxide may betransferred by a pump to refrigerating machines of a slaughterhouse, forexample. As the use of carbon dioxide for cooling replaces cooling withelectricity, the total release of carbon dioxide is reduced even thoughthe carbon dioxide after cooling is released to the atmosphere. If thereis a river available in the vicinity to which the carbon dioxide may beconducted from the cooling point where it is released to final pressurethereof and gasified again, the release of carbon dioxide to theatmosphere is further reduced due to formation of carbonates in water.However, the influence of these solutions are so small that it does notjustify the use of carbon dioxide cooling applications.

At locations where industries with cooling demands can not be provided,it is possible to cool the carbon dioxide gas to so low a temperaturethat it is transformed to cardond-dioxide ice which may be marketed as auseful product for general needs of refrigerated transport. Then, theproduction of ice together with eletricity replaces partially theseparate production of carbon dioxide ice for cooling purposes in otherplaces. In regard to the generation of electricity the reduction of therelease of carbon dioxide is total as far as such a production ofcarbon-dioxide ice replaces separate production of said ice.Carbon-dioxide ice is used also for other purposes than the refrigeratedtransport. A special application may be also to gather returned spoiledbatches of lime of which calcium lye may be manufactured. The carbondioxide in the exhaust gas of the power plant may be bound to thecalcium lye and thereby produce limestone which may be used again forproduction of lime. By this kind of operations the problem of greenhouseeffect due to carbon-dioxide release may be alleviated and spread thenegative influence to a longer period whereby the plants have a betteropportunity to make use of the release. At the same time the sufficiencyof limestone is improved which is advantageous for the futuregenerations. Today, a part of the produced lime is wasted because ofspoiling. If the recycling of limestone is not appreciated, it may beused also as a local filling material whereby no harm for theenvironment is caused. It would be a good base material for landscaping.This kind of activities are not possible in connection with thehigh-power plants because the local releases were too large. Also otherlyes could be used for absorbing the carbon dioxide component of thecombustion gases of a small-scale power plant. Thus, a small-scale powerplant provides a subject for research of the utilization of local wasteand byproducts of industry in our struggle against the greenhouseeffect. There is hardly any comprehensive solution to this problem, andso all the partial solutions amending the situation should be found. Inthis way, no remarkable further financial encumbrances are set to thisgeneration, either.

Due to a preferred small size of the optimally operating districtheating power plant of the invention, it is advantageous to design andacquire the steam power plant part by using modules of certain size,preferably two or three suitable sizes. Some examples of rating theplant and combining the modules for obtaining a desired steam plantpower are presented in FIGS. 13 to 18. Then, the most expensive part ofthe power plants could be produced in series, and the production couldbe automated, too, which would lower the investment costs. Also, therating, quality, and performance of the steam plant part would beassured, as the modules would be standard products the functionality andreliability of which could be assured.

FIGS. 13 and 14 present one module solutions. In FIG. 13, a biggermodule M1 is selected for coverage of 50 percent of the district heatingpower range. In FIG. 14, a smaller module M2 is selected for coverage ofonly about 40 percent of the district heating power range. Accordingly,the power of the peak-load engine must be higher in the solution of FIG.14 than in the solution of FIG. 13, for covering the peak part H1 of thedistrict heating power range. However, the excessive investment cost fora more powerful peak-load engine in FIG. 14 is quite small in relationto the reduction of investment costs for a smaller steam plant module.The overcapacity R1 is bigger in FIG. 13 than in FIG. 14 which meansthat more regulation is needed in the solution of FIG. 13. In summertimeit would be advantageous to purchase the needed power, which portion isshown by S1.

FIG. 15 present a solution with one bigger module M1 and one smallermodule M2, and FIG. 16 a sorresponding solution with two bigger modulesM1. The reference sign S2 indicates the need of supplementary powerneeded in autumn before starting and, respectively, in spring afterdowndriving the upper module M2, M1, respectively. This portion may bepurchased or produced with the peak-load engine. The reference sign R2indicates the overcapacity (need for regulation) of the upper module. Asthe upper module M2 or M1, respectively, are driven in the area ofintermediate load, in which the price of the electricity is higher thanin the area of base load, also competition bidding for fuels availablewithin a reasonable transport distance and somewhat more expensive thanthe fuel used for base load power generation may be arranged. Inaddition to coal, such fuels as brown coal, milled peat, or driedmunicipal waste could be considered for the lower module. For the uppermodule also wood chips, by-products of forest industries, constructionwood waste, or combustible waste could be taken into account.

In the solutions of FIGS. 17 and 18, a combination of three modulesM1/M1/M2 or M1/M2/M2 are used, respectively, involving the overcapacity(regulation need) area R3 of the third module, too.

In the plants with more than one module, selections may be made alsobetween more expensive axial turbines and less expensive throttlecontrolled turbines because normally it is enough that one turbine iscontrolled. Because both modules are operating with the same parameter,it is possible to share the need of control of the boilers betweentherebetween. This improves the efficiency of the boilers.

The electric power of the modules could be 3, 5, and 7 MW, for example.With a plant of higher power, a reasonable alternative for a diesel as apeak-load engine is a gas turbine.

The invention may vary within the scope of the accompanying claims.

What is claimed is:
 1. A method for optimally operating co-generation ofelectricity and heat in which a district heating power range is dividedinto a lower heating power range and a higher heating power range,comprising: providing base load electricity and regulation electricityproduced with a steam turbine with operating parameters of a condensingturbine, wherein producing the lower heating power range mainly by heatpumps using anergy of exhaust steam of the steam turbine as an energysource, producing peak-load power and wintertime regulation electricitywith a peak-load engine, producing the higher heating power rangepartially by heat pumps using said anergy as the energy source andpartially by exhaust gas heat of said peak-load engine; producing boththe electricity and the heat with a high fuel utilization rate and forincreasing production of electricity in relation to production of heat;producing a bigger amount of electricity of a fuel unit at an initialstage of operation of a district heating power plant; and producingextra peak power with the peak-load engine at short notice and with agood fuel utilization rate.
 2. A method according to claim 1, whereinproducing the lower heating power range includes cooling the exhauststeam of the steam turbine with a gas.
 3. A method according to claim 1,wherein the steam turbine is operated with a final pressure of expansionand a temperature level of the exhaust steam that are lower than aconventional district heating power plant but higher than a condensingturbine with seawater cooling operating in cold regions, and areselected so that a value of the anergy discharged to a condenser is ofno value and adverse effects caused to an environment by the heat goingto the sea or other water system are minimized.
 4. A method according toclaim 1, wherein the steam turbine is operated with a final pressure ofexpansion and such a temperature level of the exhaust steam that arelower than with a conventional district heating power plant and, at asite where outdoor air is cooler than seawater, at least one of equal toand lower than those of a condensing turbine with seawater cooling, andare selected so that a value of the anergy discharged to a condenser iszero and adverse effects caused to the environment are minimized becausethe anergy is released to air.
 5. A method according to claim 1, whereinproviding base load and regulation electricity includes operating withan industrial company with a high cooling demand whereby carbon dioxideis used for cooling, and is obtained by cooling combustion gases forliquifying carbon dioxide.
 6. A method according to claim 1 whereinproviding base load and regulation electricity includes using returnwater of a district heating system as an auxiliary heat source for theheat pumps.
 7. A method according to claim 6, wherein steam, afterexpansion in the steam turbine, is conducted through a new small-scaleturbine to a condenser and not to a feedwater preheater.
 8. An optimallyoperating district heating power plant for co-generation of electricityand heat, comprising: a steam power plant part with a steam turbine withoperation parameters of a condensing turbine for producing base loadelectricity and regulation electricity; a first heat pump plantconfigured to produce a lower district heating power range by usinganergy of exhaust steam of said steam turbine as an energy source; apeak-load engine configured to produce mainly peak-load power andwintertime daily peak-load electricity; means for recovering heat of anexhaust gas of the peak-load engine; a second heat pump plant using theanergy of the exhaust steam of said steam turbine as an energy sourcefor producing higher district heating power range partially by saidanergy and partially by exhaust gas heat of the peak-load engine; meansfor producing base load electricity and heat with a high fuelutilization rate and increase production of base load electricity inrelation to production of heat; means for producing greater amounts ofelectricity from a fuel unit at an initial stage of operation of thedistrict heating power plant; and means for producing extra peak powerwith the peak-load engine at short notice and with a good fuelutilization rate.
 9. An optimally operating district heating power plantaccording to claim 8, further comprising means for storing heat forshort term peak loads.
 10. An optimally operating district heating powerplant according to claim 8, further comprising means for cooling theexhaust steam of the steam turbine with gas.
 11. An optimally operatingdistrict heating power plant according to claim 8, wherein the steampower plant part includes a previous district heating power plant with alow-pressure chamber with a longer expansion for producing condensingelectricity.
 12. An optimally operating district heating power plantaccording to claim 8, wherein a boiler of an existing power plant isreplaced with a cooling unit for cooling exhaust gases that makesfeedwater preheaters needless, and steam, after expansion in the steamturbine, is directed to a small-scale turbine connected to a condenserand not to a feedwater preheater.
 13. An optimally operating districtheating power plant according to claim 8, further comprising a coolingunit for cooling combustion gases and making liquified carbon dioxide sothat the district heating power plant can provide an industrial companywith a high cooling demand that relies on liquified carbon dioxide forcooling.
 14. An optimally operating district heating power plantaccording to claim 8, wherein an existing heating power plant isprovided with the peak-load engine, exhaust gas heat recovery equipmentand heat pump solution of the optimally operating district heating powerplant, and means for using return water of a district heating system asan auxiliary heat source for the heat pumps.
 15. An optimally operatingdistrict heating power plant according to claim 8, further comprising anauxiliary cooling arrangement for production of extra electricity,wherein said auxiliary cooling arrangement is at least one of a BoostEnergy Converter and other equipment based on Rankine cycle whichproduce extra electricity by lowering the enthalpy level.